Optical disks are becoming the base for computerized archives and data bases because of their huge capacity. When they are organized in a jukebox which contains tens of disks and numerous disk drivers, they can form very large archives.
The main disadvantage of these disks and disk system is the serial mode in which the data is read from them. Coupled with slow access time this mode limits the search performance and the rate at which the data is transferred to the computing system. An optical disk contains an index of the data. By using the index, the access time to a piece of data is shorter than if the data has to be searched for serially. However, for the more demanding applications, where the same data is searched for in varying contexts or by varying criteria, the index is of no help and searching through all the disk is required. Such a search can take a very long time if done serially. Even worse, when the search is done on a jukebox which contains a few drivers and tens of disks to be searched, the search time becomes extremely long. The rate of transfer from the data to the disk to the system is much slower than the processing rate of a common CPU unit. Hence, much of the CPU time is wasted in waiting for data being fetched from the disk.
Optical disks of various kinds are based on concentric circular or spiral tracks on which the data is written in the form of spots of changes of some optical property of the disk media. These changes are typically spots of higher reflection than the non data-containing parts of the disk. These data spots lie along a spiral or circular path on the disk. The disk is divided into sectors, and each loop of this spiral path that lies within a sector is called a track. The data is read from the optical disk by an electro-optical head which projects a narrow light spot on the rotating disk, reads the reflected light from the disk and transduces it into an electrical signal. At any point in time the head reads one spot of data. By rotating the disk below the head, a track is read spot by spot. To read data from outer or inner tracks, the head is moved to the desired track by a motor. In advanced split head design the moving head is composed of a lens, a focusing coil motor, a fine tracking motor and mirrors. This combination of rotating disk with a radially moving head enables to access any data spot on the disk. The reading of the data spots is done serially along a track or along the whole spiral path. Commercially available optical disks have internal data transfer rates of about 15 Mbit/sec, which translates to an output rate of 1.25 Mbyte/sec. Searching through a whole disk may thus take more than ten minutes.
Optical heads can read several tracks simultaneously when combining a lens with a wide field of view and a matrix of light detectors that cover this field. Processing the signal from each of these detectors separately, in parallel, multiplies the data throughout proportionately to the number of detectors. Such a matrix of detectors is described, e.g., in U.S. Pat. No. 5,111,445. However, implementing parallel reading of an optical disk by a detector matrix requires first solving some severe problems.
The first problem is that of sampling the data spots that lie on a polar coordinate system by a matrix of detectors set in a rectangular coordinate system. As a result of such sampling, data spots may be sampled not in track order, which may result in the scrambling of the data structure.
Another problem is that unlike the electro-optical head according to the known art, which has a servo system to center the head over the track, centering a line-scan matrix over many curved tracks is very difficult. Without centering the detectors on the tracks, radial movement of the disk may shift the data spots between neighboring detectors and thus, again, scramble the data structure. All of this has to be done at the very fast speed of the data spots (i.e., 15 Mbit/sec.).
Yet another problem is that a detector matrix cannot economically cover all the tens of thousands of tracks, so that some means for fast moving to different group of tracks is needed. After each of such movement the detector matrix have to be aligned to the right tracks.
Generally speaking, reading data from an optical disk requires high precision. Ordinarily, such a reading is carried out using coherent light, even if writing of the data has been effected with non-coherent light, e.g., as in U.S. Pat. No. 3,805,275 or U.S. Pat. No. 4,135,251. In some instances non-coherent light has been used in illuminating data in the form of the amplitude-modulation of the relief height of a pattern of surface relief variation, such as in the microfiche-like system described in U.S. Pat. No. 4,486,870. However, such systems cannot read multitrack information and require the use of precise and expensive servo mechanisms.
It is therefore clear that it would be highly desirable to be able to overcome the aforesaid problems and to provide reading means capable of transferring to the system data from an optical disk at a high rate.
It is an object of the present invention to provide such a method and apparatus which permit to read high amounts of data and to transfer data at high rate from the optical disk to the system.
It is another object of the invention to provide a system for reading data from an optical disk which does not require mechanical adjustment to compensate for small disk drift in the radial position, and to eliminate the fine tracking coil in the moving part.
It is a further object of the invention to provide a system for fast access to any data (track) on the disk or on multiple disks.
It is still another object of the invention to provide a device by means of which a plurality of disks can be read by a single head.